Wiegand wire and similar transducers have been described in the automotive electronic literature, see for example "IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY", Vol. VT-26, No. 2, May 1977, as well as German Patent Disclosure Document DE-OS No. 26 54 775. The Wiegand effect itself has been described in "Electronics" of July 10, 1975, page 100 et seq. This effect is based on sudden reversal of polarization of the Wiegand wire upon change of a magnetic field; a coil which is in inductive coupling with the Wiegand wire then provides a sharp peaked needle induction pulse. This needle-shaped pulse permits exact determination of a switching instant, that is, correlation of a specific shaft position, for example the crankshaft position of an internal combustion engine with respect to the generation of the pulse. It does not provide an extended pulse output, however, which may be realized from example from a Hall transducer or an induction coil transducer, and which can provide not only a characteristic determinative of the ignition instant but also a characteristic which determines the closing of the ignition switch, that is, the dwell time of the ignition system. If a Wiegand wire is used, it is necessary to provide for electronic simulation of this dwell time period.
It has already been proposed to operate with two complementary impulses of a Wiegand transducer and to control a flip-flop therewith--see German Disclosure Document DE-OS No. 2 731 373--and to obtain an output signal from the flip-flop (FF) which is representative of the closing time, that is, the angular travel of the shaft during which the ignition controlling semiconductor should be closed, that is, the dwell angle. FFs are not well suited for use in ignition systems since they are sensitive to noise or stray pulses.
It has also been proposed to start a counter from a short pulse, for example derived from a Wiegand transducer, the counter then determining the closing or dwell time--see German Disclosure Document DE-OS No. 28 24 981. Two counting cycles in separate counters are needed in order to retain a predetermined, preferably fixed, closing time even under dynamic changes of engine operation. When a single counter is used, the danger may occur that ignition sparks will miss during dynamic changes, since the fixed closing time determination, as counted by the counter, could occur possibly before or even after the actually determined ignition time instant.